This invention relates generally to forced-air convection heaters and, more particularly, to a high-wattage electrical device where load variations in the powering of the device are minimized to prevent light flicker.
The electrically loading of a high-power device often results in an instantaneous voltage droop in the associated power supply. For example, when the lights momentary dim as a refrigerator turns on. These instantaneous load changes can even cause power surges which blow fuses or harm other electrical equipment on the power line. When the loading of the mains supply by a high-power electrical device changes rapidly, a noticeable flicker in the electrical lighting can occur.
A variety of consumer electronics products produce large voltage fluctuations which, at the very least are annoying and, often times disruptive to computer equipment and monitors. Some of these high-wattage electric heating devices include clothes irons, electric frying pans, skillets, woks, fondue pots, waffle irons, toasters, hair dryers, portable heaters, and electric blankets.
The disruptive effect of high-wattage electrical devices is more of a concern in medical settings. Electrical forced-air heaters are often used in keeping patients warm during an operation. However, doctors have been known to have the heaters turned off, to avoid the obnoxious effect of light flicker.
A typical forced-air warming unit consists of a blower, heater, and a temperature controller. The temperature controller moderates the power supplied to the heater so that the bulk temperature of the air exiting the warming unit, or at some other control point, is maintained at a fixed, set-point value. In general, the warming unit""s heater is sized so that its power dissipation is much greater than required to maintain a given air temperature. The large power dissipation permits the heater to meet steady-state thermal requirements within a wide range of ambient temperatures. Further, the time required to achieve the set-point temperature is minimized.
Several strategies are available for regulating the power supplied to the heater, and one of the most common is known as pulse-width modulation. Pulse-width modulation works by supplying the full supply voltage to the heater as a square wave. The duty cycle (the ratio of the on-time to the complete period) is varied by the controller so that the power supplied to the heater, averaged over time, maintains the set-point temperature.
One problem associated with the pulse-width modulation technique is the potential for extremely severe, periodic power mains loading, which occur with every transition to an xe2x80x9conxe2x80x9d cycle by the controller. In a typical warming unit, the entire heater load (approximately 0.8-1.2 kW) is switched on and off at a duty cycle which is proportional to the product of mass airflow and the required temperature difference. Switching a load of this magnitude causes a large inrush of current to flow in the power mains. Because of the power line impedance, the voltage on the power mains drops when these large current inrushes occur. This voltage drop can cause a perceptible flicker in any light connected to the same mains as the warming unit.
Several techniques are known which may be used to minimize the flicker. However, these methods all have certain drawbacks which make them unsuitable in some respect.
One technique involves reducing the switching frequency below 0.2 Hz (one transition every 5 seconds, or longer). This switching frequency appears to be a threshold below which most people do not perceive flicker. However, since the switching period is very long, it is not possible to maintain the air temperature of the warming unit within an acceptable range.
Another technique involves switching power to the heater load at a rate equal to the line frequency. This technique requires specialized circuitry which synchronizes the switching rate to that of the applied line frequency, typically between 50 and 60 Hz. This method is very effective at eliminating flicker. However, because of the relatively rapid current transition rate, this technique also generates a large amount of electromagnetic emissions which must be suppressed with expensive and massive filtering circuitry.
It would be advantageous if a high-wattage heating device could be developed that would minimize load fluctuations upon the power supply.
It would be advantageous if forced-air heating units could be developed which did not produce a noticeable flicker in the lighting. It would be advantageous if this heater were available for use in hospital settings.
It would be advantageous if a xe2x80x9cflicker-freexe2x80x9d heater could be developed that was capable of operating over a wide temperature range, and also capable of rapidly responding to the selection of a new set-point temperature, or a change in input temperature.
Accordingly, a convection heater is provided having a substantially constant load to minimize light flicker. The heater is comprised of two basic sections, a roughing (first) heater and a finishing (second) heater. The first heater uses and dissipates most of the power, continuously heating the air to a first temperature which is close to the desired output temperature. The second heater variably heats the air. The combination of the first and second heaters raises the air temperature from the first temperature to the desired temperature. In this manner, the changes in the heater loading remain relatively small.
Typically, the power dissipated by the first heater is at least twice as great as the second heater. However, the critical feature is that the peak power of the second heater is minimized, for example, to a peak power of less than 200 watts. The variable loading of such low-power element produces no noticeable light flicker.
In the simplest aspect of the invention, the first heater is a single element and the second heater is a single element. In some aspects of the invention the first heater is multitapped. Upon determination of the desired output temperature, the first heater stage is selected which closely approaches, but does not exceed, the set-point. The second heater is then used to make up the difference between the heat supplied by the first heater and the desired output temperature. In some aspects of the invention, the first heater stages are dynamically varied to more quickly approach the target temperature, and the minimize the difference in heat that must be applied by the second heater.
In one aspect of the invention, the first and second heaters are both comprised of a plurality of heater sections. That is, the first heater is a first combination of heater sections selected from the plurality of heater sections, and the second heater is a second combination. Each of the plurality of heater sections dissipates a different peak power level, where the difference is graduated in steps of less than approximately 200 watts. Depending on the desired operating temperature and ambient conditions, the heater controller selectively activates each of the heater elements. A first peak power is generated by the first heater throughout a timed cycle, and a second peak power level intermittently occurs during the cycle. The first and second power levels are dynamic, so that the absolute values of the first and second peak powers may change with every cycle.
A method for regulating the loading of a high-wattage power device is also provided. The method comprising:
continuously dissipating a first peak power into a medium; and
periodically dissipating an additional peak power into the medium.
The additional peak power is selected to be small. As a result of dissipating the first peak power, an output temperature is generated that is approximately the desired medium temperature. As a result of periodically adding an additional peak power, the desired temperature is generated.